1. Field of the Invention
The present invention relates to an overcurrent protection device for protecting a circuit to be protected from overcurrents by melting a low-melting element or tripping a PTC element.
2. Description of the Related Art
In conventional practice, current fuses composed of low-melting metal elements such as lead, tin, antimony, and the like and designed to evolve heat and to melt by an overcurrent are known as protection elements designed to prevent overcurrents from reaching circuits to be protected.
PTC elements are also known as such overcurrent-suppressing protection elements. A PTC element is a resistor element made of barium titanate or another inorganic component or obtained by dispersing electroconductive particles in a crystalline polymer material (for example, a polyolefin-based resin). When in an overcurrent mode, the element heats up, its resistance value increases, and the current flowing through the circuit to be protected is suppressed.
In another type of protection device, as shown in FIG. 11, FETs are connected in series with the circuit to be protected, an IC for detecting the voltage drop due to the FETs is also provided, the FETs are energized by the IC when the voltage drop due to the FETs exceeds a given value, and the FETs shuts off the current flowing through the circuit to be protected.
Typically, however, a fuse does not melt immediately after the current being carried exceeds the rated current, but does so only after a current somewhat greater than the rated current has been sustained for some time. For example, the UL standards define fuses as products which xe2x80x9cmelt within 60 seconds after a current twice the rated current flows.xe2x80x9d The result is that, for example, a fuse rated a 3 A does not melt at less than 3 A and that there are as yet no products that would melt as a result of self-heating immediately after a level 1 mA above the 3 A has been reached.
Thus, the difference between the rated current and the shut-off current (current flowing during the melting of the fuse under actual conditions) is considerable in some fuses, and these fuses do not melt immediately after the actually flowing electrical current has exceeded the rated current. For some applications, such fuse characteristics are completely unacceptable.
PTC elements have the same drawback. Specifically, a PTC element will not trip unless it carries a current at least twice the rated current (non-tripping current value).
By contrast, a protection device obtained by combining an IC and FETs in the manner shown in FIG. 11 has a smaller difference between the rated and shut-off currents than in fuses or PTC elements. Because FETs are semiconductors, however, they are sometimes damaged by the application of voltage or electromagnetic waves, and this damage, although not a particular safety concern during current shut-off, is highly dangerous when the system is powered.
For safety reasons, protection devices should preferably be actuated not only when an overcurrent flows but also when ambient conditions (optical, magnetic, or dynamic) or external conditions (temperature, humidity, or the like) change abnormally for any reason.
It is an object of the present invention to overcome the above-described shortcomings of prior art and to provide an overcurrent protection device that is highly safe and is capable of rapidly shutting off or suppressing the electric current when a current exceeding a prescribed current value flows or when an abnormality is detected in the external operating environment.
Striving to attain the stated object, the inventors perfected the present invention upon discovering that an effective solution would be to heat a low-melting metal element or PTC element with a heating element rather than by self-heating under overcurrent conditions and to allow rapid heat evolution when the voltage drop quantity in the power line leading to the circuit to be protected exceeds a prescribed value or when an outside sensor for detecting abnormalities in the external operating environment detects an abnormality, and thus to provide a first detector element for detecting the voltage drop quantity in the low-melting metal element or PTC element or to provide an outside sensor for detecting abnormalities in the external operating environment, and also to provide a switching element for abruptly passing a large electric current through the heating element when the first detector element detects a prescribed voltage drop quantity or when an abnormality is detected by the outside sensor.
Specifically, the present invention provides an overcurrent protection device, comprising:
a substrate;
formed thereon a heating element and at least one of a low-melting metal element and a PTC element, the low-melting metal element being melted or the PTC element being tripped by the heat of the heating element;
a first detector element for detecting the voltage drop in the power line leading to a circuit to be protected, or an outside sensor for detecting abnormalities in the external operating environment; and
a switching element for passing a large electric current through the heating element and rapidly heating the heating element in accordance with the voltage drop quantity of the first detector element or the signal from the outside sensor.
In particular, a device in which an IC is used as the first detector element and in which an FET is used as the switching element is provided as such an overcurrent protection device.
These and other objects, features and advantages of the present invention are described in or will become apparent from the following detailed description of the invention.